In recent years, deep ultraviolet LEDs with a light emission wavelength of around 265 nm have attracted attention for use in a variety of applications, such as sterilization and water purification. FIG. 22 is a cross-sectional view illustrating an exemplary structure of a typical conventional deep ultraviolet LED. In the LED illustrated in FIG. 22, light emitted from a quantum well layer 5 is emitted in the upward direction (toward the air) via a barrier layer 4, an n-AlGaN layer 3, an AlN buffer layer 2, and a sapphire substrate 1. At this time, part of the light is totally internally reflected due to the difference in the refractive index among the n-AlGaN layer 3, the AlN buffer layer 2, the sapphire substrate 1, and the air in accordance with the Snell's law, and the reflected light then travels in the direction toward an Al (or Au) reflecting electrode layer 11, but the light is almost entirely absorbed by a p-GaN contact layer 9 or a Ni layer 10 and thus is lost inside the LED.
Meanwhile, light emitted from the quantum well layer 5 and propagating in the downward direction is also absorbed by the p-GaN contact layer 9 or the Ni layer 10 and thus is lost almost entirely.
Therefore, with the structure illustrated in FIG. 22, more than 50% of light is lost inside the LED. At this time, the external quantum efficiency (EQE) is about 5% and the light extraction efficiency (LEE) is about 10%.
Patent Literature 1 discloses providing a projection/recess structure on an upper surface or a side surface of a sapphire substrate in order to suppress total internal reflection and improve the light extraction efficiency by about 20%.
Meanwhile, as a new method for improving the light extraction efficiency, there has been introduced a technique of forming a photonic crystal periodic structure, which has a period of about equal to the wavelength of light, in a light extraction layer. The photonic crystal periodic structure is formed at the interface between two structures with different refractive indices, and typically has projections and recesses mainly made of pillar structures or hole structures. In a region where such a periodic structure is formed, the existence of light is prohibited and thus total reflection is suppressed. It is known that using such a structure can improve the light extraction efficiency (see Patent Literature 2).
In addition, non Patent Literature 1 indicated below has reported that replacing a p-GaN contact layer, which absorbs deep ultraviolet light, with a p-AlGaN contact layer, which is transparent to deep ultraviolet light, and further forming a Ni layer as thin as possible, for example, to a thickness of about 1 nm can improve the light extraction efficiency by 1.7 times.